Conventionally, there has been provided a drilling apparatus which includes a weight hammer provided with a compression spring for biasing the weight hammer, and which hammers an object to be hammered with a hammering force generated upon falling of the weight hammer. According to this apparatus, a hammering force obtained by adding a biasing force of the compression spring to the gravity of the weight hammer is generated. Therefore, even if a falling distance of the weight hammer is short, a large hammering force can be obtained.
The inventor of the present invention has provided, as such drilling apparatus, the following drilling apparatus (for example, see Patent Document 1). Specifically, the drilling apparatus includes: a weight hammer provided to be slidable; a compression spring for applying the biasing force to the weight hammer; rotors each provided with an engagement claw which engages, through rotation thereof, the weight hammer to cause the weight hammer to move from an original position to a direction of increasing the biasing force of the compression spring, and which releases the engagement of the weight hammer at a predetermined position. In the drilling apparatus, through rotation of the rotors, the engagement claw engages the weight hammer to cause the weight hammer to move to the direction of increasing the biasing force of the compression spring, and releases the engagement of the weight hammer at the predetermined position to cause the weight hammer to move to a direction of the original position with the gravity of the weight hammer and the biasing force of the compression spring, and thus the weight hammer hammers an object to be hammered (shank rod) with the hammering force obtained by adding the biasing force of the compression spring to the gravity of the weight hammer.
The drilling apparatus has a structure in which the weight hammer is lifted up from a lower side thereof by the rotors and is caused to fall. Therefore, the engagement portions of the rotors engage the weight hammer to lift up the weight hammer from the lower side thereof. When the engagement portions pass through a top dead point, the engagement portions are detached from the weight hammer and then the weight hammer is caused to fall. In this case, driving means (for example, motor) of the rotors receives a large load when the weight hammer is engaged and lifted up from the lower side thereof. Further, a phenomenon where, when the weight hammer is detached from the engagement portions, the above-mentioned load is eliminated in an instant repeatedly occurs, and an inertial force of the rotors, which is generated when the above-mentioned load is eliminated in an instant, is repeatedly applied to the driving means. Therefore, the driving means is adversely affected, and durability of the driving means is decreased.
Further, the drilling apparatus has a structure in which, by rotating the rotors, the engagement portions of the rotors engage the weight hammer to lift up the same from the lower side thereof and the engagement portions are detached from the weight hammer in a vicinity of the top dead point and then the weight hammer is caused to fall, to thereby hammer the object to be hammered (shank rod). Therefore, the number of times of hammering per a unit time can not be sufficiently increased. For example, in a case where one engagement portion is provided to the rotors, the weight hammer performs hammering one time for one revolution of the rotors. In a case where two engagement portions are provided to the rotors at 180° intervals, when the rotors semi-rotates, the weight hammer performs hammering one time (hammering two times per one revolution). In this manner, the number of times of hammering per a unit time is determined depending on the number of engagement portions to be provided to the rotors and rotating speed of the rotors. In the conventional structure, there are naturally limits of the number of engagement portions to be provided to the rotors and also rotating speed of the rotors. Therefore, there is also a limit of increasing the number of times of hammering per a unit time, and hence the number of times of hammering per a unit time can not be sufficiently increased.
In this regard, the inventor of the present invention has provided the following drilling apparatus in Patent Document 2. Specifically, in the drilling apparatus, a plurality of weight hammers are provided. The rotors for driving the weight hammers are provided correspondingly to the weight hammers, respectively. The rotors are provided in such a positional relation that the engagement claw of one rotor engages and drives one weight hammer, and, when the engagement of the weight hammer is released, the engagement claw of another rotor in driving engages another weight hammer. The rotors are driven by the same driving means.
With this, each of the rotors is provided in such a positional relation that the engagement claw of the one rotor engages and drives the weight hammer, and, when the engagement of the weight hammer is released, the engagement claw of the another rotor in driving engages the another weight hammer. The rotors are driven by the same driving means.
Therefore, even in a case where the one rotor is driven to engage and move the one weight hammer and the weight hammer is detached from the engagement claw of the one rotor at a predetermined position, the engagement claw of the another rotor engages the another weight hammer. Therefore, the driving means of the rotors receive always a certain load of causing the weight hammer to move (drive). Thus, without applying an excess load due to rapid change of load to the driving means (for example, motor), the durability of drilling apparatus is improved.
Further, the plurality of weight hammers are provided, and hence it is possible to greatly increase the number of times of hammering per a unit time.    Patent Document 1: JP 2005-023551 A    Patent Document 2: JP 2007-239323 A